Protective circuits for rectifiers



March 6, 1945. J. DYSON 2,371,035

PROTECTIVE CIRCUIT FOR RECTIFERS Filed Aug. 9, 1943 Inventor" James Dyson,

H is. Attorney.

Patented Mar. 6, i945 PROTECTIVE cmcm'rs roa nnorlrmns James Dyson, Rugby, England, assignor to General Electric Company, a corporation of New York Application August 9 1943, Serial N0. 497,946

In Great Britain ctober'22, 1942 3 Claims.

My invention relates to rectifier circuits and in particular to rectifier circuits in which harmful voltages can'arise across the rectifier if the unidirectional current circuit thereof is opened. It is an object of my invention to provide an improved protective circuit for such a rectifier circuit.

In U. S. Letters Patent No. 2,153,378, Kramer, granted on April 4, 1939, and assigned to the same assignee as the present invention, there is disclosed a direct current measuring arrangement in which genuine transformer action isobtained and which comprises a saturable inductance carrying direct current windings and alternating current windings, the latter being supplied with alternating current from an alternating current source through a full wave rectifier. The direct current output of the rectifier flows through a measuring instrument whose current capacity, because of the transformer action of the measuring arrangement, need not be of the same order as that of the circuit in which current is to be measured. In such an arrangement it is often convenient to locate the measuring instrument becomes operative upon the arising of greater than normal voltages across the rectifier terminals to prevent the rising of these voltages to values greater than that which the rectifier is designed to withstand.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however,

both as to its-organization and methodof operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. l is a direct current measuring arrangement embodying my invention; Fig. 2 is a modification of the current measuring arrangement of Fig. 1; and Fig. 3 is a schematic diagram representing another modification of my invention. Throughout the figures of the drawing, like reference characters are utilized to designate like I parts.

In the arrangement illustrated in Fig. 1, there is direct current winding which may be in the form of a single rod or bar ll carryin'g a direct current which is to be measured, and, in inductive relation to the direct current windin II, is a pair of magnetic cores l2 and I3. The cores l2 and I3 are preferably laminated or are stripwound from magnetic strip material and they are shown as continuous ringssurrounding the direct current winding or rod ll. Each of the cores l2 and I3 carries an alternating current winding and, preferablmthe windings are divided so that the core l2 carries a plurality of alternating current windings l4 and the core l3 carries a plurality of alternating current windings IS. A source of alternating current I5 is provided to which the alternating current windings of the cores E2 and I3 are connected in opposition through the full wave rectifier H. The multiple group of windings it may be connected in series with the multiple group of windings IS.

The fullwave rectifier l'l consists of four rectifier units is which are arranged in the wellknown bridge, or Graetz, connection and a direct current responsive device is connected across the opposite points 2|, 22 of the rectifier. The direct current responsive device may take, for example, the form of a DArsonval galvanometer.

A current transformer 24 is connected in series with the alternating current circuit l6 and supplies current to an auxiluiary. rectifier 25 consisting of four rectifier units 26 connected similar to the units ofrectifier l1. Connected across the output terminals 21, 28 of rectifier 25 is a load resistance 29 whose opposite terminals 39, 3! are connected respectively to the opposite terminals 32, 33 of the direct current responsive device 20, terminal 30 being connected to terminal 33 through a half wave rectifier 34.

As .is disclosed in the above-mentioned patent to Kramer, the cores I2 and I3 are composed of a material having asubstantially rectangular magnetization curve and the design is such that the core material is operated in a substantially. horizontal portion of the magnetization curve. One such material is an alloy, sold under the trade name Mu-Metal, which consists of approximately 74% nickel, 20% iron, 4% copper, 1

chromium, and lesser amounts of manganese and silicon.

In the operation of the direct current transformer portion of the above described circuit, the v at a minimum, as well winding ll, of course, carries the direct current to be measured. Magnetic cores IE2 and i3 and, windings l4 and I5 are so designed that the magnetic material operates in the substantially horizontal portion of the magnetization curve. Each of the cores l2 and i3 develops half of the flux wave and together they give rise to a square wave alternating current across the input terminals of rectifier H. The magnitude of the voltage of the source |6.is without the magnitude of the current flowing .through the windings l4 and I5 for the reason that the current in these secondary windings is governed only by the amount of compensating fiux needed to overcome the effect of the primary or direct current. This square wave alternating current is rectified by the rectifier I1 and flows through the current responsive device 23.

In such a circuit it is evident that the voltage across the rectifier units |3 is only that due to the voltage drop caused by current flowing through the device 2|! and the leads connecting this device to the terminals 2|, 22. This voltage may be, for example, of'the order of volts whereas the full alternating current voltage of the source I6 is of the order of 250 volts. The rectifier accordingly is designed to withstand voltages of the order of 10 volts, although of course it may have a normal voltage rating slightly higher in order to maintain reverse current through the rectifier as to withstand high frequency transient voltages which occur in normal operation of the transformer.

It is apparent that should the device 29 be accidentally disconnected from across the terminals 2|, 22 the full alternating current voltage is impressed across the rectifier units. In order to prevent this full voltage from appearing across these terminals to puncture or burn out the individual units I8, an alternative path is provided for the unidirectional currents of the rectifier I! in the case of such an accident. This path comprises the resistance 29 which is connected across the points 32, 33 of the metering circuit. Physi cally, the point 32 may comprise terminal 2| and the point 33, terminal 22.

If the resistance of the device and the leads connected thereto between R. and the unidirectional current provided by rectifier I1 is I, the voltage across the terminals 32, 33 is IR. Similarly, if the direct current provided by rectifier 25 is 11 and the resistance of resistor 29 is R1, the voltage across the terminals 30, 3| is I1R1. The net voltage, therefore, around the circuit 30, 3|, 32, 33 is IRI1R1. If the ratio appreciable eifect upon the units of rectifier I1 the points 32 and 33 is of the transformer 24 and the magnitude of resistance 29 be chosen so that I1R1 is greater than IR, the net voltage around the above-mentioned circuit is negative and tends to "force current through rectifier-34 in the non-conducting direction. .The resultant current which flows through this rectifier is, of course, very small so that thepotential drop is equal to R1(I+I1). Taking a to keep this current to a minimum actress numerical example, if R=l ohm, R1=l..5 ohms, and I=I1=5 amps, the potential drop across resistance 29 is 1.5 (5+5) =15 volts. The potential across rectifier 34 may be voltage across rectifier I1 is then'20 volts. Since may readily be designed to withstand such a voltage, the meter 20 may be disconnected indefinitely without damage to the rectifier units.

In the protective circuit for the direct current measuring arrangement shown in Fig. 2, improved accuracy of the measurements is obtained by the cormection of a small inductance 40 in series with rectifier 34. When inductance 40 is not present in the circuit, the voltage wave across the points 32, 33 differs slightly from that across the points 30, 3| by reason of the inductive reactance of the leads connected to the metering device 23 between the terminals 32, 33. This difference in wave form gives rise to a small current through rectifier 34 at certain points of the cycle. Insertion of inductive reactance or choke 40 is effective value and to improve the accuracy of measurements.

In-the modification of the invention shown in Fig. 3, a plurality of direct current circuits 4|, 42, 43 are supplied with individual current measuring arrangements 44, 45, 46, the output currents of which flow through a single totalizing metering device 41. The current measuring arrangements 44-46 may be located at relatively distant points and the circuits may be supplied respectively with alternating voltages from different sources 48, 49, 50. The construction of arrangements 44-46 is similar to that of the arrangement shown in Fig. 1, with the exception that the rectifiers 25, instead of being supplied with current from a series type of transformer, are connected across the respective secondary winding of transformers 5| connected across the alternating current sources 48-50. By such an arrangement the protective circuit of each individual metering arrangement is rendered independent of the current flow in the other direct current circuits. Thus, if a current type of transformer is used for the current flow in one of the direct current circuits 4 l-43, the voltage drop across the corresponding resistance 29 would be lower than the potential drop across the rectifiers I! of the other measuring circuits and the resistance 29 of th metering arrangement having the low load condition would act as a shunt path across the metering device 41 to give a false indication of total current flow. Use of the transformers 5|,

however, prevents this condition by providing a constant current 12 from the rectifiers 25 in the circuits 44-46. V

The current measuring arangements 44-46 differ from that of Fig.

1 also in that an inductive reactance 53 is connected in series with the resistance 29 in each of the arrangements 44-46 to function as a smoothing choke for the current 12 flowing through cuits of the rectifiers 25. The potential drop across the resistance 29 in the arrangements 44-45 ischosen such that it is greater than the voltage which can arise across the rectifiers I1 in the absence of an open circuit in the direct current paths 4l-43. In order to provide correct summation of currents flowing in the remaining circuits 44-45 when the alternating current supplied to one of these circuits is switched off while the others are operating, the half wave rectifiers 55 are provided between the terminals 33 and the metering device 41 in each 5 volts so that the total the current measuring the direct current cirof the metering arrangements. Thus, for example, if the A. C. source 48 is switched 01!, the

rectifier 55 inmetering arrangement 44 prevents the fiow of the output currents of circuits and 46 through theshunt path across device 4i'con1- prising resistance 29, coil 40, and'half wave rectifier 34 in the metering circuit 44. r

The operation of the remaining portions of the metering arrangements 44-46 of the circuits of Fig. 3 is the same as that described in connection with the circuit of Figs. 1 and 2. It is apparent, of course, that the presence of transformer 5| connected across the alternating voltage source renders the expression for the maximum voltage across the rectifiers I! of the circuits more complicated than in the case oi the measuring arrangementsof Figs. 1 and 2. It is apparent, also, that metering devices may be used for the individual circuits 44-45 to indicate the current flowing through the direct current circuits 4l-43.

While I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a source of alternating potential, a rectifier having input terminals connected to said source through an impedance and output terminals across which a unidirectional potential is developed, an output circuit connected across said output terminals, said impedance limiting the intensity of the potential across said rectifier when said output circuit is closed, and means to limit the intensity of the potential across said rectifier when said output circuit is opened, said means comprising a resistance and a unilateral conducting device connected in series across said output terminals, and means to cause a unidirectional current to flow throughsaid resistance to develop thereacross a voltage having an intensity greater than said unidirectional potential, said voltage being of such polarity and said unilateral device being so poled that current from said rectifier is prevented from flowing through said resistance when said out- 4 'being so poled that current from said first rectifier is prevented from flowing through said resistance when said unidirectional circuit is closed.

- 3. In combination, a source of alternating potential, an inductive load and a rectifier connected in series circuit across said source, saidrectifier having a unidirectional output circuit, and means for preventing rise of potential across said rectifier upon the opening of said unidirectional circuit, said means comprising a second rectifier coupled to said series circuit, a resistance connected across the unidirectional current output circuit of said second rectifier, and means including a half wave rectifier connecting said resistance across the output circuit of said first rectifier, said second rectifier and said half wave rectifier being so poled that current from said first rectifier does not fiow through said resist 

